1. Field of the Invention
The present invention relates to a torque detection method for detecting the torque of the output shaft of a wave gear device. More particularly, the invention relates to a wave gear device torque detection method that uses a strain gauge affixed to the flexible external gear of a wave gear device to utilize the elastic deformation of the flexible external gear to detect the output shaft torque with good precision.
2. Description of the Prior Art
A wave gear device is composed of an annular rigid internal gear, a flexible external gear disposed inside the rigid internal gear and a wave generator disposed inside the flexible external gear that flexes the external gear radially to partially mesh the flexible external gear with the rigid internal gear and circumferentially rotate the meshing positions. The wave generator generally has an elliptical profile that flexes the flexible external gear into an elliptical shape. When the wave generator is rotated by a motor, the positions at which the two gears mesh move around the circumference, generating a relative rotation arising from the difference in the number of teeth. Therefore, by fixing one of the gears, a reduced-speed rotational output can be obtained from the other gear.
One method of detecting the output torque of the wave gear device is to utilize the elastic deformation of the flexible external gear. As shown in FIG. 1, a cup-shaped flexible external gear 1 comprises a cylindrical body 2, an annular diaphragm 3 whose inner edge is continuous with an end of the body 2, and an annular boss 4 formed continuously from the center part of the diaphragm 3. External teeth 5 are formed integrally with the outer surface at the open end of the body 2 to run in the circumferential direction. The torque of the output shaft can be detected based on the outputs of strain gauges affixed to the cylindrical body 2 or diaphragm 3.
However, each part of the flexible external gear 1 is forcibly and repeatedly radially deformed by the flexing and rotation action of the wave generator. This generates strain in the flexible external gear 1 unrelated to the transmission torque. For each rotation of the wave generator, each part of the flexible external gear 1 undergoes two radial deformations having a fixed amplitude. Thus, with each rotation of the wave generator, this produces a two-cycle distortion of the sinusoidal fundamental period (each cycle being 180°), unrelated to the transmission torque.
In the prior art, as shown in the example of FIG. 1, a strain gauge group f1(p) (R1, R2) and a strain gauge group f2(p) (R3, R4) are placed on the surface of the flexible external gear 1, for example on the surface of the diaphragm 3, with a mutual offset of 90°, and the outputs used to cancel the fundamental period strain component. However, with this method the detection output does not have sufficient linearity and the detection output is left with rotational ripple components having short periods (that are an integral multiple of the fundamental period).
To cancel the fundamental period strain component and a secondary strain component (90°, cycle), the arrangement shown in FIG. 2 has been used in which a set of the strain gauges of groups f1 (R1, R2) and f2 (R3, R4) are disposed at 45° to each other, and another set of the strain gauges of groups f3 (R5, R6) and f4 (R7,R8) are disposed at 45° to each other in a manner that these two sets are disposed at 90° to each other, whereby the eight strain gauges R1 to R8 are placed on the diaphragm. To improve the output linearity, an arrangement also includes using sets of strain gauge groups (f1, f2) and (f3, f4) arranged symmetrically within 360°, for a total of sixteen strain gauges.
However, even with these configurations, rotational ripple compensation is still inadequate. Based on their experiments, the present inventors found that the main causes of such error are strain gauge positional errors, flexible external gear dimensional errors (asymmetries and the like), and gear assembly error. Errors in the positioning of the strain gauges, for example, give rise to differences in strain gauge signal sensitivity and phase that make it impossible to cancel or adequately control the rotational ripple.
An object of the present invention is to provide a wave gear device torque detection method that is able to remove or adequately control periodically fluctuating strain components (rotational ripple) produced in the flexible external gear with the rotation of the wave generator that are unrelated to transmission torque.